Species-Specific Light-Energy Allocation Helps Explain the Relationship Between SIF and GPP in Two Plantation Ecosystems

Solar-induced chlorophyll fluorescence (SIF) is a powerful proxy for photosynthesis, yet its quantitative relationship with gross primary productivity (GPP) varies considerably among ecosystems. The underlying mechanisms for this variability, particularly in structurally complex forests, remain poorly understood. Using tower-based SIF and eddy-covariance measurements from two contrasting plantation forests—a drought-tolerant Quercus variabilis and a fast-growing poplar—we investigated how species-specific light-energy allocation strategies influence SIF-GPP relationships and the performance of mechanistic light-reaction models. The key findings are as follows: (1) The photochemical mechanistic model substantially reduced the underestimation of GPP under high-light conditions. A simplified qL-PAR model, derived from leaf-level active fluorescence measurements, produced comparable performance to that of the full photochemical model, i.e., the MLR-NPQ method—a mechanistic light-response model with canopy SIF. (2) The MLR model captured seasonal light-energy partitioning dynamics across two plantations and revealed contrasting light-energy allocation patterns between the two plantation systems. Poplar prioritized photochemical efficiency (ΦP, averaging 0.41) to maximize carbon gain, whereas Q. variabilis invested more energy in NPQ and SIF for a photoprotective “safety-first” strategy. (3) However, MLR models exhibited inconsistent performance. For the poplar plantation, the model achieved higher accuracy (R2 = 0.83 at the daily scale and R2 = 0.65 at the half-hourly scale) than for the Q. variabilis plantation (R2 = 0.65 at the daily scale and R2 = 0.45 at the half-hourly scale). (4) The contrasting ΦP–ΦF relationships provide a physiological explanation for the species-specific performance of the SIF-based GPP models. For poplar, ΦP–ΦF maintained a tight linear relationship across a wide range of moderate-to-high PAR levels. In contrast, for Q. variabilis under high PAR (ΦP = 0.249), NPQ became saturated while SIF continued to rise, leading to a decoupling of the ΦP–ΦF relationship. Our results suggest that the contrasting light-energy allocation patterns observed at the two sites may contribute to the performance discrepancies in canopy-scale SIF-based GPP simulations. Our results collectively validate that a photochemical mechanistic framework advances the process-oriented simulation of photosynthetic energy partitioning in plantation canopies. These findings highlight the potential need to integrate species-specific ΦP–ΦF relationships to better constrain the competitive balance between carbon gain and photoprotective safety—a key uncertainty that must be reconciled for robust GPP retrievals over managed forests but requires further validation. These findings contribute to a mechanistic understanding of SIF–GPP relationships in tall forests and provide a reference for improving vegetation parameterization in regional-scale modeling applications where such ecosystems are represented.